Kaposi's sarcoma-associated herpesvirus (KSHV) or human herpesvirus 8 (HHV8) is the most recently described DNA tumor virus. It is the infectious trigger for Kaposi's sarcoma, body cavity-based primary effusion lymphomas (PEL), and some subtypes of multicentric Castleman's disease (CD) for review see (37), KSHV-related CD is a polyclonal B cell hyperplasia that is presumably driven by KSHV vIL-6 secretion as well as other viral proteins. In contrast, PEL are B cell lymphomas that generally have a monoclonal origin as determined by immunoglobulin gene rearrangement and viral terminal repeat analyses (7, 20, 36). Terminal repeat analyses by Judde and colleagues (20) have also demonstrated that KS tumors can have an oligo- or monoclonal pattern, and may evolve from a polyclonal hyperplasia into a monoclonal tumor. Thus, KSHV may contribute to cell proliferation through secretion of viral cytokines and induction of cellular cytokines as in the case of CD, as well as through expression of transforming viral oncogenes, particularly in the case of PEL.
The KSHV genome has significant sequence homology to all classes of herpesviruses, but is unique among the human herpesviruses in encoding an extensive number of regulatory genes which have been pirated from the host genome during its evolution (30, 36). While a number of these genes have homology to known cellular oncogenes or transform rodent cell lines in vitro (2, 14, 26), only a small number of KSHV genes are routinely found to be expressed in tumor tissues. vBCL-2, vIRF1, vGPCR, and K01 are examples of KSHV proteins which might contribute to cell transformation in vitro but are not appreciably expressed in most KSHV-infected KS or PEL tumors (21, 24, 32, 38).
KSHV infected PEL cell lines constitutively express three viral genes, vFLIP (ORFK13), vCYC (ORF72), and LANA1 (ORF73), which are not inducible by tetradecanoyl phorbol acetate (TPA) or inhibited by phosphonoformic acid (PFA) and thus are unambiguously designated as latent or class I genes. These three proteins are transcribed on the major polycistronic latent transcripts, LT1 and LT2 (10, 39, 42). In vitro studies demonstrate that the viral cyclin associates with cyclin dependent kinase (CDK) 4 and 6, and phosphorylates pRB (8, 16, 28). LANA1 is believed to bind to the origin of replication to tether the viral genome to host chromatin during mitosis, effecting equal segregation of viral genome during division (3). LANA1 also binds to p53 and inhibits p53-mediated transcriptional activity and apoptosis (13). vCYC over-expression induces apoptosis (31) and it is at least theoretically possible that this may be inhibited in situ by the anti-apoptotic activities of other latency expressed proteins, such as vFLIP and LANA1.
Viral protein expression is highly restricted in KS and PEL tumors. Presently, only LANA1 protein has been shown by immunohistochemistry to be expressed in situ in all cells infected by KSHV (11, 22, 32). Viral cyclin and ORFK12 transcripts have been identified by in situ hybridization in all KSHV infected cells (9, 34), however, protein localization has yet to be performed. No other viral proteins examined thus far, including vIL-6 (K2), minor capsid protein (ORF26), K8, K8.1, vIRF1 (K9), K10, K11, PF-8 (ORF59), and ORF65 have a similar in situ constitutive pattern of expression (21, 32).
KSHV gene expression studies remain controversial. Since PEL cell lines can be manipulated into lytic replication by TPA and butyrate, studies on cultured cell lines have been used to classify KSHV genes into mutually-exclusive latent and lytic classes based on transcription kinetics (40). Frequently, KSHV expression patterns from cultured cell studies are assumed to be similar in tumor tissues in situ without direct evidence. However, a number of KSHV genes are expressed at low levels in resting PEL cell lines but are induced to high expression levels during TPA treatment and thus have properties of both latent and lytic genes (analogous to the EBV LMP1 expression pattern). This pattern of gene expression has been referred to as class II expression (37). Recent studies demonstrate that extension of results from expression studies in tissue culture cannot be uniformly applied to human tumor tissues in part because KSHV may have tissue-specific gene expression patterns. vIL-6, for example, behaves as a class II protein in tissue culture cell lines and is expressed in hematopoietic-derived cells but generally not in KS lesions (29). Thus, determining precisely which viral genes are likely to play a role in KSHV-related pathogenesis requires direct tissue examination of each tumor type. <Discovery of additional genes that are constitutively expressed in KSHV-induced disorders is particularly important since these genes are likely to play a role in cell growth dysregulation.
For these reasons, discovery of a KSHV gene having a tissue-specific expression profile is important, particularly if the encoded protein is functionally capable of contributing to cell proliferation. In this paper we describe a new KSHV gene (K10.5) expressed in KSHV-infected hematopoietic tissues. This gene is located in a region containing a cluster of viral sequences with limited homology to the interferon regulatory factor (IRF) family of proteins (36). vIRF1 is encoded by ORF K9 and inhibits interferon-induced transcription and fully transforms NIH3T3 cells (12, 14, 27, 44). vIRF1 binds to histone acetyltransferase transcriptional coadaptors (5, 19) and induces cell transformation by activating the cMYC oncogene through an interferon-stimulated response element (ISRE) called the PRF element (19). Based on these findings and the fact that other tumor viruses target the same tumor suppressor pathways as KSHV, Jayachandra et al. found that both Epstein-Barr virus (EBV or HHV4) EBNA2 and adenovirus E1A proteins also activate cMYC but use differing sets of coadaptors from those used by vIRF1 (19). vIRF1 additionally inhibits p53- and Fas-induced apoptosis ((5) and unpublished obs, S. Jayachandra, P. S. Moore, Y. Chang). vIRF1; however, is not generally expressed in PEL or KS and is therefore unlikely to contribute to these diseases although it may be important in the pathogenesis of CD (21, 32). Another IRF-like KSHV open reading frame encoding vIRF2 and having NF-kB-inhibitory activity has been described (6). We show here that LANA2 is a B-cell specific factor that antagonizes p53 tumor suppressor functions and is expressed during latency.
KSHV/HHV8 is associated with three proliferative diseases ranging from viral cytokine-induced hyperplasia to monoclonal neoplasia: multicentric Castleman's disease (CD), Kaposi's sarcoma (KS), and primary effusion lymphoma (PEL).
Here we report a new latency-associated 1704 bp KSHV spliced gene belonging to a cluster of KSHV sequences having homology to the interferon regulatory factor (IRF) family of transcription factors. ORFK10.5 encodes a protein, latency-associated nuclear antigen 2 (LANA2), which is expressed in KSHV-infected hematopoietic tissues including PEL and CD, but not KS lesions. LANA2 is abundantly expressed in the nuclei of cultured KSHV infected B-cells. Transcription of K10.5 in PEL cell cultures is not inhibited by DNA polymerase inhibitors nor significantly induced by phorbol ester treatment. Unlike LANA1, LANA2 does not elicit a serologic response from patients with KS, PEL or CD as measured by Western-blot hybridization. Both KSHV vIRF1 (ORFK9) and LANA2 (ORFK10.5) appear to have arisen through gene duplication of a captured cellular IRF gene. LANA2 is a potent inhibitor of p53-induced transcription in reporter assays. LANA2 antagonizes apoptosis due to p53 overexpression in p53-null SAOS-2 cells and apoptosis due to doxorubicin treatment of wild-type p53 U2OS cells. While LANA2 specifically interacts with aminoacids 290-393 of p53 in glutathione-S-transferase pull-down assays, we were unable to demonstrate LANA2-p53 interaction in vivo by immunoprecipitation. These findings show that KSHV has tissue-specific latent gene expression programs and identify a new latent protein which may contribute to KSHV tumorigenesis in hematopoietic tissues via p53 inhibition.